1. Field of the Invention
The present invention relates to an image processing device and an image processing method.
2. Description of Related Art
Image defects such as errors, missing and blots have been detected by reading a printed sheet and checking the obtained image against a checked reference image.
In order to reduce false detection due to a slight displacement of an image, defects have been detected such that a feature point having a unique image characteristic is detected from both images, and the feature value thereof representing the image characteristic is checked.
For example, an edge is detected from an image to be checked as a feature point, and the edge direction and edge intensity of the edge are determined as the feature value (e.g. see JP 2007-156626A).
On the other hand, the FAST (features from accelerated segment test) is known as a method that detects a corner as a feature point (e.g. see Machine learning for high-speed corner detection, Edward Rosten, Tom Drummond). The FAST utilizes machine learning so that a software can detect a feature point at a high efficiency and a high accuracy. That is, the FAST is optimized in order to increase the speed of the software detection.
However, determining the feature value of the edge direction requires filtering operations individually for vertical, horizontal and diagonal directions with respective edge detecting filters, which results in the increased number of operations. Further, since the feature value of the edge intensity has large data volume, a large storage volume is required for storing the feature value, which results in an increased cost. Furthermore, another problem is that false detection is likely to be increased when a noise is included in the surrounding pixels of a target pixel.
In contrast, the FAST is less influenced by a noise since a feature point is determined based on the surrounding pixels that are located in a circular pattern at a distance of several pixels from a target pixel. Accordingly, a feature point having a unique image characteristic can be determined at a high accuracy. However, the FAST is only a technique for detecting a feature point. To obtain a feature value, an additional step of calculating a unique feature value of the feature point is required.